Process for the protection of magnesium and magnesium base alloys against corrosion



Patented June 20, 1944 PROCESS FOR THE PROTECTION OF MAG- NESIUM ANDMAGNESIUM BASE ALLOYS AGAINST CORROSION Charles James Bushrod,Prestwich, England, as-

signor to Magnesium Elektron Limited, London,

England, a company of Great Britain No Drawing. Application April 5,1943, Serial No.

. 481,952. In Great Britain April 2, 1942 3 Claims.

This invention relates to processes for protection of magnesium andmagnesium base alloys against corrosion by treatment of the metal by asolution which reacts chemically with the metal to produce a protectivefilm on the surface of the metal. Two such processes are widely used inGreat Britain, viz. the 30 minute hot chromate bath and the cold acidchromate bath, described in paragraphs 3 (iii) and 3 (iv) of D'I'D911and more fully described in the Handbook "Elektron Magnesium Alloys 1937pages 91-95. The 30 minute bath whilst giving good surface finishes hasthe two disadvantages of lengthy period of treatment and necessity formaintaining the solution near to boiling point. The acid chromatetreatment however involves a cold solution and a very short period ofimmersion but the solution is strongly acid and therefore removes asmall but appreciable amount of the metal (of the order of 0.0005" totalreduction in thickness)- during treatment and is therefore not sosuitable for use on parts machined to fine tolerances.

Th process described in paragraph 3 (i) in D'I'D911 involves the use ofelectrical equipment and a period of treatment of 20 minutes and theprocess described under paragraph 3 (ii) involves ganese dioxide bytreatment in solutions at normal atmospheric temperatures and containingsodium hypobromite, potassium permanganate, or other suitable oxidisingagent.

The initial film can be produced by immersion in a solution at normalatmospheric temperature and having the following features:

(1) An acid reaction with a pH value of less than 5 and preferably lessthan 4.

(2) One or more sulphates, nitrates, or chlorides, or similar activatingsalts are in solution.

(3) The solution should contain ions'of one or more elements which formsparingly soluble hydroxides e. g., chromium, aluminium,-iron, ormanganese, or alternatively, compounds'which can be reduced to producesuch ions e. g., dichromates and permanganates.

(4) The solution should be well buii'ered so that the pH will vary onlyslowly with the area of metal treated to ensure a long life for thesolution.

The treatment may be in accordance with the invention described inBritish Patent No. 510,353

a cold solution but requires a rather lengthy period of treatment, viz.minutes; and these processes form the subject of British Patents Nos.493,935 and 510,353.

All four processes produce films on the metal which are somewhat porousand the degree of protection afforded to the metal and of the adherenceof the film to the metal depends to some extent at least on the degreeof porosity. The main object of the present invention is to produceprotective films having a greater degree of con-.

. is used in its broader meaning viz. the addition to a compound of anelectronegative atom or radicle. I

I have found that manganese compounds in the film are particularlysuitable and for instance manganous hydroxide can 'be oxidised tomanprovided that the solution produces a film containing a compoundwhich can be oxidised.

I have found that a solution containing 20-30 grams per litre of potashalum and 20-80 grams of MnSO4.5I-I2O per litre of solution will producean oxidisable film after an immersion of three minutes. In theseparticular experiments sheets of Elektron (registered trade-mark) AM503alloy consisting of about 1.5% manganese balance magnesium were used.The sheets were subsequently 4 drained for about 10 seconds and thendipped whilst wet in an aqueous solution containing 5% NaBrO.

Another solution capable of producing an oxidisable film containsmanganese sulphate, ammonium sulphate, and potassium dichromate e. g.80, and 40 grams respectively per litre of water.

The oxidising solution should, of course, be incapable of attacking themetal to any serious extent. The oxidising eflect can be observed tosome extent from the colour of the films which changes from light brownproduced by the first solution to a deep chestnut colour after immersionin th second solution. If desired the effect of the oxidising solutionmay be accelerated by maintaining the solution above normal atmospherictemperature e. g. the potassium permanganate solution may be at C.

The formation of the film and/or its oxidation may be effected orfacilitated if desired by anodic or cathodic action.

In addition to successful tests on magnesium base alloys containingsubstantially only manganese as an alloying constituent, I ha e alsosuccessfully treated alloys containing substantial proportions ofaluminium e. g. 6 to 9%. I claim: 1. A process for the protection ofmagnesium and magnesium base alloys against corrosion, comprising firsttreating the metal with a solution, said solution containing principallymanganous sulfate and being capable of forming a film of a manganouscompound on the surface of the metal, then treating the metal with anaqueous solution containing principally sodium hypobromite to oxidizethe manganous compound film.

2. A process for the protection of magnesium and magnesium base alloysagainst corrosion,

comprising first treating the metal with a solution, said solutioncontaining principally man- 20 assaovs ganous sulfate and being capableof forming a film of manganous hydroxide on the surface of the metal,then treating the metal with an aqueous solution of an oxidizing saltcapable of oxidizing the manganous hydroxide to manganese dioxide.

3. A process for the protection of magnesium and magnesium base alloysagainst corrosion, comprising first treating the metal with a solution,said solution containingprincipally manganous sulfateand being capableof forming a film of a manganous compound on the surface of the metal,then treating the metal with an aqueous solution containing principallysodium hypobromite to oxidize the manganous compound film, such firstsolution also having incorporated therein ammonium sulfate and potassiumdichromate.

CHARLES JAMES BUSHROD.

